This invention relates generally to electronic musical instruments and more particularly to an improved tone generator system for such instruments.
A currently popular tone frequency oscillation generator for use in electronic organs is the system described, for example, in U.S. Pat. No. 3,236,931, known as the "top octave synthesizer" (TOS), which is driven by a stable master oscillator and is operative to generate the 12 notes in the top octave of an electronic organ, that is, from C.sub.7 (4186.009 Hz) through B.sub.7 (7902.133 Hz). A plurality of frequency divider chains, one for each note produced by the TOS, are used to derive the notes for the lower octaves of the instrument. An important advantage of this tone generator is its amenability to fabrication in integrated circuit form, and since all of the frequency divider chains (usually 12 in number) are of identical construction, the system is relatively easy to construct at low cost. Only one stable master oscillator is required, further contributing to reduction in cost, and since all of the tone signals generated by the system are locked together, the whole organ can be tuned by simply adjusting the master oscillator; that is, when the frequency of the master oscillator is adjusted, every note in the whole organ follows exactly. Accordingly, if the environment of the master oscillator is controlled (temperature, humidity, etc.), there is little chance of the oscillator drifting in frequency, and once properly tuned, there is little tendency for the organ to go out of tune. However, because all octaves of a given note are phase-locked, whenever notes of a given nomenclature (C-C#-D, etc.) are sounded from any key on the instrument, all such notes will be dead in tune with one another.
In a live musical performance, the pleasant, exciting chorus effect comes about by reason of the use of multiple instruments; that is, a plurality of individual performers whose instruments are not always in tune with each other, but which average out in a random fashion to produce a pleasant effect. Similarly, a pipe organ is rich in chorus effects by reason of its many ranks which, due to the different effect of the environment on each cannot be kept in perfect tune with each other, but vary in random fashion to cause heating between signals of the same nominal frequency. This might suggest that chorus effects could be achieved by using two TOS tone generator systems of the kind described above, with one master oscillator tuned slightly from the other. The results of this approach have been found to be less than completely satisfactory, however, because in such a system all of the notes derived from one tone generating system are out of tune with the corresponding notes of the other system by exactly the same percentage and the result is not at all equivalent to the beating obtained from a truly random beating pattern.
One approach to obtaining a tone generator system with chorus effects is described in applicant's U.S. Pat. No. 3,538,234 wherein a full-range tone generator operates together with a second, short-range tone generator which includes only a range of frequencies over which the sensitivity of the human ear to ensemble or chorus effects is substantial, namely, in the mid-octave range. The notes from the short-range generator are each produced by an independently tunable oscillator which can be tuned away from the frequency of the corresponding note from the full-range generator by the amount necessary to achieve the desired chorus effect. Some drift in frequency between the individual oscillators can be tolerated, but since the components used in the oscillators are usually temperature sensitive, and in the interest of keeping their cost down, their environment is not controlled, there is a tendency for all of the oscillators to change in frequency with temperature sufficiently to require re-tuning to maintain the requisite frequency difference between nominally corresponding notes of the full-range and short-range generators. Moreover, should it be necessary to re-tune the full-range generator, obviously all of the individually tunable oscillators would have to be separately re-tuned to restore the proper frequency relationships between the two generators.